This invention relates to a magnetic resonance imaging (MRI) apparatus and, more particularly, an MRI apparatus which employs a gradient coil unit disposed in a vacuum vessel for noise reduction.
An MRI apparatus detects magnetic resonance signals from nuclei (e.g. proton) in an object and reconstructs a tomographic image of a desired slice position of the object. An MRI apparatus holds a great advantage in medical use because of non-invasiveness or non X-ray dosing.
In general, an MRI apparatus comprises a cylindrical gradient coil unit inside a magnet unit including a cylindrical superconductive magnet. The superconductive magnet contained in a cryostat filled with cooling medium is disposed inside a hollow cylindrical vacuum vessel. Inside the gradient coil unit a radio-frequency (RF) coil is disposed.
According to a predetermined pulse sequence, the RF coil applies a radio-frequency field and the gradient coil unit applies a gradient field to an object. Subsequently a magnetic resonance phenomenon occurs in the object and a magnetic resonance signal is detected by the RF coil. A magnetic resonance (MR) image is obtained by reconstructing the detected MR signal.
The gradient coil unit includes three channels of gradient coils producing gradient fields along x, y and z axis, respectively. These three gradient coils are switched frequently and rapidly. When switching, the gradient coils and the magnet vessel supporting the gradient coil unit vibrate. This vibration causes a shock sound. This shock sound echoes in an imaging bore which a patient is placed and the patient feels uncomfortable, anxious or fear.
To reduce this noise, a number of arrangements have been offered. For example, gradient coils are covered by soundproof materials. However, this is not successful to reduce the noise sufficiently.
U.S. Pat. No. 4,652,365 discloses an MRI apparatus in which gradient coils are disposed inside a vacuum vessel for maintaining the superconductive condition of a superconductive magnet to prevent the noise to transmit outside the vessel. This arrangement requires the breakage of the vacuum when adjusting the positions of the gradient coils after the MRI system is assembled. Moreover, when cooling medium for the gradient coils leaks in the vacuum vessel, the vacuum condition gets worse which causes evaporation of the cooling medium of the superconductive magnet.
Japanese patent application No. 62-79889 describes an MRI apparatus which includes a first vacuum vessel for a superconductive magnet and a second vacuum vessel, in which gradient coils are disposed, coaxially provided in a bore of the first vacuum vessel. This arrangement requires a complex manufacturing process and causes a low cost-performance because two separate vacuum vessels are provided. Moreover, the imaging bore can not be sufficiently large and the patient feels oppression in the imaging bore.